用户自定义变量
用户自定义变量格式为 @var_name。var_name 目前只支持字母，数字，_$组成。用户自定义变量是大小写不敏感的。
用户自定义变量是跟 session 绑定的，也就是说只有当前连接可以看见设置的用户变量，其他客户端连接无法查看到。
用 SET 语句可以设置用户自定义变量：
SET @var_name = expr [, @var_name = expr] ...
或
SET @var_name := expr对于 SET 语句，赋值操作符可以是 = 也可以是 :=
例：
mysql> SET @a1=1, @a2=2, @a3:=4;
mysql> SELECT @a1, @a2, @t3, @a4 := @a1+@a2+@a3;
+------+------+------+--------------------+
| @a1 | @a2 | @a3 | @a4 := @a1+@a2+@a3 |
+------+------+------+--------------------+
| 1 | 2 | 4 | 7 |
+------+------+------+--------------------+如果设置用户变量用了 HEX 或者 BIT 值，TiDB会把它当成二进制字符串。如果你要将其设置成数字，那么需要手动加上 CAST转换: CAST(.. AS UNSIGNED)：
mysql> SELECT @v1, @v2, @v3;
+------+------+------+
| @v1 | @v2 | @v3 |
+------+------+------+
| A | 65 | 65 |
+------+------+------+
1 row in set (0.00 sec)
mysql> SET @v1 = b'1000001';
Query OK, 0 rows affected (0.00 sec)
mysql> SET @v2 = b'1000001'+0;
Query OK, 0 rows affected (0.00 sec)
mysql> SET @v3 = CAST(b'1000001' AS UNSIGNED);
Query OK, 0 rows affected (0.00 sec)
mysql> SELECT @v1, @v2, @v3;
+------+------+------+
| @v1 | @v2 | @v3 |
+------+------+------+
| A | 65 | 65 |
+------+------+------+
1 row in set (0.00 sec)如果获取一个没有设置过的变量，会返回一个 NULL：
mysql> select @not_exist;
+------------+
| @not_exist |
+------------+
| NULL |
+------------+
1 row in set (0.00 sec)用户自定义变量不能直接在 SQL 语句中被当成 identifier，例：
mysql> select * from t;
+------+
| a |
+------+
| 1 |
+------+
1 row in set (0.00 sec)
mysql> SET @col = "a";
Query OK, 0 rows affected (0.00 sec)
mysql> SELECT @col FROM t;
+------+
| @col |
+------+
| a |
+------+
1 row in set (0.00 sec)
mysql> SELECT `@col` FROM t;
ERROR 1054 (42S22): Unknown column '@col' in 'field list'
mysql> SET @col = "`a`";
Query OK, 0 rows affected (0.00 sec)
mysql> SELECT @col FROM t;
+------+
| @col |
+------+
| `a` |
+------+
1 row in set (0.01 sec)但是有一个例外是如果你在 PREPARE 语句中使用它，是可以的：
mysql> PREPARE stmt FROM "SELECT @c FROM t";
Query OK, 0 rows affected (0.00 sec)
mysql> EXECUTE stmt;
+------+
| @c |
+------+
| a |
+------+
1 row in set (0.01 sec)
mysql> DEALLOCATE PREPARE stmt;
Query OK, 0 rows affected (0.00 sec)更多细节